BIOLOGY 205/SECTION 7 DEVELOPMENT- LILJEGREN Lecture 8 Development of the Fruit Fly Drosophila 1. The fruit fly- a highly successful, specialized organism a. Quick life cycle includes three larval stages and a metamorphosis. Spends only 1 day as an embryo b. Body plan built from a repeated set of units: segments. Sets of genes control development in segmented animals (including us) by specifying and refining segment fates. i. Each segment has unique identity! ii. Head 3 segments. Thorax- 3 segments. Abdomen-8 segments. iii. True for maggot and adult. 2. Why flies? a. Lots of reasons we talked about the first day of class: small, sequenced genome, many genetic and molecular tools, small and cheap. b. Very reproducible anatomy- every hair same from one fly to next, and can see segmentation on outside of body c. Anatomical, developmental, & even behavioral similarities to vertebrates. Adult flies do many things that we do - they eat, sleep, learn, have sex etc. So these processes can be genetically analyzed in flies. d. Primary reason- History i. In early 1900s when science of genetics rediscovered, Thomas Hunt Morgan chose fruit fly to study genetics. He was one of first scientists to systematically isolate "mutations" affecting visible traits (at first just found naturally occuring mutations). A “mutant” is an animal lacking the product of single gene. The first fly mutant found in 1910 was the white mutant. 1. Normal, wild-type flies have eyes containing red pigments that protect vision. 2. Found mutant fly with white eyes. 3. White gene encodes a transmembrane channel protein that pumps pigment precursors into cells. It turns out that it’s a member of a large family of channel proteins. 4. Related to a human gene that when mutated causes cystic fibrosis! This related CFTR protein is a transmembrane channel protein that moves chloride ions. ii. Thousands of mutations have now been identified that affect all aspects of body structure/function. Christiane Nüsslein-Volhard and Eric Wieschaus set up a huge screen to look for developmental mutants that affect the body plan of fruit flies - they got lots of mutants and a Nobel prize! The analysis of these mutations has increased our knowledge of development dramatically. e. Today, we’re going to talk about some of the keys stages of embryonic development and pattern formation in the fly—remember that the fly’s body plan is assembled in 24 hours!3. The Maternal Effect Genes Establish the Anterior-Posterior axis early in development. a. These genes, like bicoid and nanos, are mRNAs made by nurse cells and pumped into the oocyte through cytoplasmic bridges. They form gradients which set up the Anterior-Posterior axis. The proteins are mostly transcription factors that activate GAP GENES (see below). b. Mutations in these genes are called maternal effect mutants because the genotype of the mother determines the phenotype of the offspring. 4. Let’s think about one of these maternal effect genes, bicoid, as an example. a. bicoid mRNA localized to anterior end. Upon fertilization mRNA is translated into protein which forms a gradient—lots of Bicoid at anterior end, some in middle, and none at posterior end. bicoid encodes transcription factor- i.e. it turns other genes on or off. i. Remember, these proteins and RNAs are used by the embryo during cleavage, when nuclei are dividing but cell membranes haven’t yet formed. Therefore, gradients can form easily. b. Different genes are activated by different levels of Bicoid i. Some genes only turned on where there is lots of Bicoid. ii. Other genes only turned on by moderate amounts of Bicoid iii. Other genes turned off by Bicoid--therefore only turned on where bicoid is absent. iv. Thus this gradient of Bicoid leads to the next set of genes being activated in broad regions of the embryo. These expression domains, in turn, can interact to define new subdivisions c. How do we know this? i. Experiment #1- Remove Bicoid from embryo i.e. bicoid mutant embryo 1. In absence of Bicoid anterior region of body missing (embryo has no head and thorax). ii. Gap genes regulated by bicoid also encode transcription factors e.g. Hunchback, Kruppel, Knirps. a. Hunchback is expressed in anterior region where high bicoid. b. Kruppel is expressed in central region of embryo where moderate bicoid c. Knirps expressed in posterior stripe where less bicoid. d. Hunchback maternal mRNA distributed throughout oocyte, but its translation is inhibited in posterior end AND additional transcription of Hunchback stimulated by bicoid…so its both a maternal effect gene and a gap gene 5. The GAP GENES Set Up the First Crude Embryonic Subdivisions a. Gap genes initially identified through genetics. Embryos mutant for a particular gap gene are missing entire regions of the body plan that match where that gap gene is active. Ie. kruppel mutant larva missing thorax segments and part of the abdomen. b. Gap gene proteins then turn on further downstream genes (the PAIR-RULE GENES). i. Some genes are turned on by hunchback -- some genes by Kruppel ii. Other genes are turned on only where both proteins present (where they overlap) 6. The PAIR-RULE GENES, such as Even-skipped, help the embryo form more refined segments. Most pair rule genes are expressed in 7 stripes or every other parasegment (Parasegments are found in the embryo, later larval and adult stages have segmentswhich are built on the basis of the parasegment patterning ie. a segment contains parts of 2 parasegments. See Figs 9.27 and 9.33). Most pair-rule genes encode transcription factors that control expression of SEGMENT POLARITY GENES like engrailed and wingless that we’ve talked about previously. 7. The Segment Polarity genes define anterior/posterior regions of segment a. Remember wingless and engrailed define the anterior vs. posterior of segments, by specifying which cells will have a posterior (hairless) fate. i. wingless expressed only in third cell of each segment ii. engrailed only expressed in fourth cell of each segment b. For example, engrailed is activated in cells that have high levels of the pair-rule transcription factors Even-skipped or Fushi tarazu. Wingless is activated in cells that receive no Even-skipped or Fushi tarazu but contain another pair-rule protein not shown here—Sloppy-paired. c.
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